1. Field of the Invention
The invention relates to a new and useful DC chopper circuit for use with electrical energy storage systems. More specifically, it is a new type of DC chopper circuit useful for eliminating harmonic voltages across a current source in a superconducting magnetic energy storage system.
2. Description of the Related Art
Multi-phase DC chopper circuits typically are connected in parallel between a voltage source and a current source. Each phase of the chopper has a pair of switches and a small inductor. By selectively switching the switches on and off, the current source or voltage source may be regulated, depending on the application.
DC chopper circuits with equal numbers of phases on each side of the current source are known in the art.
Examples of known multi-phase choppers having an equal number of phases on each side of the current source are found in the following patents.
U.S. Pat. No. 3,588,664 to Akamatsu teaches a DC chopper circuit with equal phases.
U.S. Pat. No. 4,748,397 to Ogawa et al. discloses an equal phase DC chopper utilizing a current controller.
Huang, U.S. Pat. No. 5,181,170, is for a DC converter type of DC chopper which uses alternate switches, wherein one side of the chopper is on when the other side phases are off. This patent also utilizes an equal number of phases on each side of the chopper circuit.
And U.S. Pat. No. 5,111,378 to Nowak et al., discloses a standard DC chopper utilizing duty switches to control each phase.
U.S. Pat. No. 3,088,040 to Newhouse discloses a super conductor switch that is essentially a multi-phase DC chopper with equal phases on each side of the chopper.
U.S. Pat. No. 4,464,617 to Yantovsky et al., discloses a multi-phase machine which uses a DC chopper type circuit having equal size phases to control it.
Dahler et al., U.S. Pat. No. 5,204,548, teaches a storage circuit utilizing a conventional DC chopper transfer energy.
A known type of multi-phase chopper consists of p single-phase switch choppers (switches), such as gate turn off (GTO), Insulated Gate Bipolar Transistor (IGBT), or other devices, connected in parallel between a current source (CS), such as a Superconducting Magnetic Energy Storage (SMES) magnet, and a voltage source (VS), such as a capacitor. Each single-phase chopper has a pair of switches and small inductors. When a multi-phase chopper operates at high frequencies, it acts as a voltage divider that reduces the voltage across the current source (CS) to a fraction of the voltage source (VS) voltage. If a p-phase chopper operates only at the discrete duty cycles (d=fraction of switch ON time) of 0/p, 1/p, . . . , p/p, harmonic voltages across current source (CS) can be eliminated. This is important for a device like superconducting magnetic energy storage (SMES) where harmonic voltages generate large AC losses and possibly induce large dynamic voltages inside the coil.
The number of discrete DC steps which can be obtained using a DC chopper with equal phases on each side is determined as follows.
If a p-phase chopper operates at the duty cycle of d=n/p (n=0, 1, 2, . . . p), the voltage across the current (CS) source is: EQU V.sub.CS =.alpha.V.sub.VS, .alpha.=(2d-1).
and the chopper current is: EQU I.sub.VS =.alpha.I.sub.CS
Also, it should be noted that: EQU VS Power=V.sub.VS .multidot.I.sub.VS =.alpha.V.sub.VS I.sub.CS =V.sub.CS .multidot.I.sub.CS =CS Power
This mode of operation, where both sides are switching between ON and OFF, allows p+1 DC voltage steps: ##EQU1## Two other modes of operation have been proposed. One side of the chopper is either always OFF (d.sub.1 =0) or ON (d.sub.1 =1) while the other side operates at d.sub.2 =n/p(n=0, 1, 2, . . . p): EQU one side is OFF: ##EQU2## EQU one side is ON: ##EQU3## All three operations can yield 2p+1 discrete DC steps: ##EQU4##